Stl Testing.pdf

MEASUREMENT OF SOUND "TRANSMISSION LOSS BY SOUND INTENSITY

A P r e l i m i n a r y Report by R.W. Guy and A. De Mey C e n t r e f o r Bu i l di n g S t u d i e s Concordi a U n i v e r s i t y , Montreal

ABSTRACT The sound i n t e n s i t y t e c h n i q u e i s being implemented t o measure sound t r a n s m i s s i o n l o s s a t t h e Cent r e f o r B u i l d i n g S tu d i e s a c o u s t i c s t e s t f a c i 1i t y . The use of t h e i n t e n s i t y t e c h n i q u e f o r t h i s purpose i s being i n v e s t i ­ g a t e d i n t h r e e main a r e a s ; v a l i d a t i o n w i t h r e s p e c t t o s t a n d a r d t e c h n i q u e s ; d e t e r m i n a t i o n of a p p r o p r i a t e measuri ng p r o c e d u r e ; e x p l o i t i n g t h e a n a l y t i c a l c a p a b i l i t i e s of t h e t e c h n i q u e . This paper p r e s e n t s some p r e l i m i n a r y f i n d i n g s with r e s p e c t to t h e s e a r e a s .

SOMMAIRE Dans l e l a b o r a t o i r e d ' a c o u s t i q u e du Cent r e des é t u d e s sur l e b â t i m e n t , l a t e c h n i q u e de mesure de l ' i n t e n s i t é a c o u s t i q u e e s t i n t r o d u i t e a f i n de d é t e r m i n e r l a t r a n s m i s s i o n du son. A c e t e f f e t , l ' u t i l i s a t i o n de c e t t e t e c h n i q u e e s t examinée dans l e s t r o i s domaines s u i v a n t s : v a l i d a t i o n p a r r a p p o r t aux normes, d é t e r m i n a t i o n d ' u n e p r o c é d u r e de mesure a p p r o p r i é e e t ^ e x p l o i t a t i o n des c a p a c i t é s a n a l i tiq u e s de^cette^méthode. Cet a r t i c l e p r é s e n t e q u e l q u es c o n c l u s i o n s p r é l i ­ m i n a i r e s à c e t ég ar d .

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1.

INTRODUCTION

T r a d i t i o n a l l y t h e sound t r a n s m i s s i o n l o s s o f a p a n e l o r wal l h a s been measured usi ng t h e s t a n d a r d , c l a s s i c approach as d e s c r i b e d by t h e ANSI/ASTM E90-81. However, t h e nuner ous c o n t r a d i c t i o n s bet ween r e p o r t e d r e s u l t s based on t h i s method [ 1] s u g g e s t s t h e need f o r f u r t h e r i n v e s t i g a t i o n and t h i s i s b e i n g a c h i e v e d t h r o u g h t h e a p p l i c a t i o n o f t h e Sound I n t e n s i t y Technique at t h e Cent r e f o r Bu i l d i ng S t u d i e s a t Concordi a U n i v e r s i t y , Montreal. This new method h a s s e v e r a l a d v a n t a g e s , f o r exampl e: i t g i v e s t h e t r a n s m i s s i o n l o s s d i r e c t l y w i t h o u t havi ng t o make c o r r e c t i o n s f o r t h e panel a r e a and t h e a b s o r p t i o n o f t h e r e c e p t i o n room; i t e l i m i n a t e s t h e e f f e c t o f f l a n k i n g t r a n s m i s s i o n ; no r e s t r i c t i o n s ar e p l a c e d on t h e c h a r a c t e r i s t i c s of t h e r e c e p t i o n room, t h a t i s i t n e i t h e r h a s t o be r e v e r b e r a n t o r a n e c h o i c ; t h i s f a c t e l i m i n a t e s t h e need of an a c t u a l t r a n s m i s s i o n l o s s s u i t e , a l t h o u g h c u r r e n t l y t h e e x i s t a n c e o f a t l e a s t o ne r e v e r b e r a n t chamber i s exploited. As opposed t o p r e s s u r e , i n t e n s i t y i s a v e c t o r q u a n t i t y and t h e r e f o r e provides d i r e c t i o n a l inform ation. In o r d e r t o me a s u r e t h e t r a n s m i t t e d i n t e n s i t y t hr o u gh a s u r f a c e , on l y t h e component p e r p e n d i c u l a r t o t h e s u r f a c e i s needed. However, t o d e s c r i b e t h e power f l o w d i s t r i b u t i o n , d i r e c t i o n or t r a n s m i s s i o n , t h r e e d i r e c t i o n a l powerflow may be d e t e r mi n e d . The r e l a t i v e c o n t r i b u t i o n s t o t h e t o t a l sound t r a n s m i s s i o n o f d i f f e r e n t s e c t i o n s of t h e t e s t panel can be d e t e r mi n e d . T h i s paper p r e s e n t s t h e e v a l u a t i o n p r o c e d u r e employed t o implement t h e Sound I n t e n s i t y Technique. The t e c h n i q u e was appl i e d t o t h e me as ur e ment o f Sound Tr a n s mi s s i o n l o s s t h r ou g h a panel wi t h and w i t h o u t a b s o r b e n t l i n e d r e v e a l ; t h e r e s u l t s t h u s o b t a i n e d were t h e n compared w i t h t h o s e o b t a i n e d us i n g t h e s t a n d a r d appr oach. In a d d i t i o n t h e e f f e c t of t h e l i n i n g was s t u d i e d and t h e d i s t r i b u t i o n o f t h e i n t e n s i t y r a d i a t e d t h r o u g h t h e p an el d e t e r mi n e d .

2.

METHODS TO MEASURE THE SOUND TRANSMISSION LOSS

The t r a n s m i s i o n l o s s i s g i v e n by:

TL - 10 l oq1()

(!,/ît )

(1)

where î ^ i s t h e i n c i d e n t i n t e n s i t y and f ^ t h e t r a n s m i t t e d i n t e n s i t y .

2.1

S t a n d a r d Approach

The s t a n d a r d method of meas ur i ng t h e Sound Tr a n s mi s s i o n l o s s of a panel o r wal l i n v o l v e s t h e u s e o f two v i b r a t i o n - i s o l a t e d r e v e r b e r a t i o n chambers t h a t ar e s e p a r a t e d p a r t i a l l y or c o m p l e t e l y by t h e p a r t i t i o n t o be studied. The t r a n s m i s s i o n l o s s i s t h e n : TL = Lp s - Lp r + 10 l o g 1Q

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(S/A)

(2)

where Lps and Lpf are re s p e c tiv e ly th e average sound pressure le v e ls (dB) in th e source and re c e iv in g rooms, S (m2) th e p a r t i t i o n ' s surface area and A (m ) the absorption of the receiving room. I t is assumed that the sound f i e l d s in both rooms are d iffu s e and that there i s no fla n k in g transmssion. 2.2

Sound In t e n s it y Approach

The determination of the transmission loss of a panel or wall is now done through the d i r e c t determination o f both the i n t e n s i t y in c id e n t on and transmitted through the te s t p a r t i t i o n . The incident i n te n s ity I- can be calculated from the measured spaceaveraged sound pressure Pfms in the source room assuming the sound f i e l d is d iffu s e [ 3 ] .

where p i s the density of a ir and c the speed of sound in a ir . The accuracy o f t h i s equation has been v e r i f i e d by (rocker et al [3] by the d ir e c t measurement of the i n t e n s i t y through the aperture formed afte r removal o f the t e s t p a r t i t i o n . From equation (3) the fo llo w in g re la tio n s h ip between the in c id e n t i n t e n s i t y level L^- and the space averaged sound pressure level Lpm can be derived [ 4 ] .

The transmitted i n t e n s i t y I I t

is measured on the receiving side of

the panel as the i n t e n s i t y v e c to r 's component perpendicular to the panel's surface. The sound transmission loss is then calculated from: TL ‘ LPra - 6 - L I t where L j t i s th e tra nsm itte d i n t e n s i t y l e v e l .

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3. TEST FACILITIES 3.1

Transmission Loss S uite

The transm is sion loss s u it e o f the Centre f o r B u ild in g Studies (C .B .S )at Concordia U n i v e r s it y c o n s ists of 2 re c ta n g u la r rooms of d i f f e r i n g dimensions. The la r g e r room, the source raom ' f o r a l l the repo rted experiments has a volume of approximately 94m . The smaller room, the r e c e iv in g room in t h i s case has a volume o f about 32 m . The t e s t aperture between the rooms has an area of 7.5m and the f a c i l i t y is shown in Fig. 1.

Fig. 1 General Layout o f Transm issio n Loss Suite a t the Centre fo r Building Studies, Concordia University.

The Schroder c u t - o f f frequency is 250 Hz f o r the la r g e r room and 400 Hj f o r the smaller one.

D iffu s in g elements c o n s is t in g o f one r o t a t i n g and two s t a t i o n a r y d i f f u s e r s were located in the source room, and fo u r s ta t io n a r y d i f f u s e r s were lo c a te d i n th e r e c e p t io n room. The t e s t f a c i l i t y i s described in d e t a i l by Lang et al [ 6 ] .

3.2

Test Wall

In o r d e r t o accomodate t h e panel s i z e t e s t e d , a heavy f i l l e r w all was constructed in the t e s t aperture between the two rooms. The composition o f th e w a ll i s g iv en in Fig. 2.

Fig. 2 Cross Section of Filler Wall at Bese Plate

As can be seen the f i l l e r wall co n sis ts o f two w a l l s , mounted one in each room on t h e i r r e s p e c t iv e room's a p e rtu re and separated from each o th e r by i n s u l a t i o n m a t e r ia l . The STC value o f the complete f i l l e r wall was 60. The t e s t panel was cm. (15.5") deep reveal splayed at 45° towards remaining w all depth. displayed in Fig. 3.

mounted flu s h to the source room, leaving a 39.4 on th e r e c e i v i n g s id e . Tine aperture was f u r t h e r the recep tio n room to minimize the e fect of the Hie method o f i n s t a l l a t i o n o f th e t e s t panel i s

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Fig.3

The t e s t panel 0 . 6 4 cm ( i" ) t h i c k .

M ounting

of

T® st

Pan»l

on

Filler

Wall

used was a 1 . 1 4 m x 1 . 1 4 m.

(45" x 45")

glass

panel,

During t h e e x p e r i m e n t s r e p o r t e d h e r e , t h e r e v e a l on t h e r e c e i v i n g s i d e was l e f t e i t h e r bare or l i n e d wi t h a 2 . 5 4 cm ( 1 " ) , 5 . 0 8 cm (2") and 1 0 . 16 cm (4") a b s o r b e n t m a t e r i a l : Gonafl ex-F (by B l a c h f o r d ) . For m a t e r i a l p r o p e r t i e s , s e e Table 1. TASIE 1.

Absorption C o e f f i c i e n t s of Conaflex F as Supplied by t h e Ma nufacturer.

Absorption C o e f f i c i e n t (%)* Freq. (Hz ) 125 250 500 1000 2000 4000

F-100 (1") 5 19 57 88 96 87

F-200 (2") 28 68 90 98 98 96

*Test method ASTM C 423-66 Te st sample s i z e 72 s q u a re f e e t ♦Approximate v a l u e s d e r i v e d from c h a r t ♦A b s o rp t i o n c o e f f i c i e n t f o r Conaflex F-400 (4") not a v a i l a b l e

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4. TEST PROCEDURE 4.1

Standard TL Measurement

White noise was generated in the source room by two loudspeakers placed in the corners of the room opposite the te s t aperture. The mean sound pressure levels in the source room were measured using a r o t a t i n g microphone boom (B & K 3923). Hie microphone described a plane c i r c u l a r path at 70° from the horizontal and the length of the arm was 1.6 m, t h i s c o n fig u ra tio n was chosen so t h a t th e microphone cleared the w alls and s ta tio n a ry d iffu s e rs by at least 0.8 m (1/4 wave length at the 125 centre frequency is 0.68 m). The minimum distance from the microphone to speakers was 1 m. and the period o f a complete re v o lu tio n o f the microphone was 32 seconds. In the receiving room, the mean sound pressure level measurment was performed in the same manner as in the source room, however because o f i t s smaller size, the length o f the arm was changed to .95 m and the tu rn ta b le was t i l t e d at 60° from the h o riz o n ta l. The reverberation time in the re c e iv in g room was calcula ted from the averaged decay points (16 per second and 50 decay samples) with the tu r n ­ ta b le in the same p o s itio n as described above and with the microphone ro ta tin g . A lin e a r regression analysis was used in the range -5 dB below th e upper decay po in ts down to 10 dB above background l e v e l . All measurements were computer c o n tr o lle d and fed to a t h i r d octave analyser. In t h i s case, the Sound In te n s ity Analyser type 2134/3360 from Bruel and Kjaer. 4.2

Sound In t e n s it y Method

The incident i n t e n s i t y was calculated from the mean sound pressure le ve l as measured by the reverberant room method described e a r l i e r . The tra nsm itte d sound i n t e n s i t y was measured d i r e c t l y using th e B & K Sound In t e n s ity Microphone Probe type 3519, using the fac e -to -fac e micro­ phone c o n fig u ra tio n . The microphones w ith 12 mm spacer were chosen which gives a useful frequency range, of 125 to 5 k with an accuracy o f +_ ldB assuming a monople source The in t e n s i t y radiated through the panel was measured at 5.08 cm. (2") behind the surface employing an a rray o f 81 evenly d i s t r ib u t e d p o in ts over the surface; t h i s choice of measuring parameters w i l l be discussed la t e r . I t became obvious during th e p r e lim in a r y t e s t phase t h a t in th e presence o f the reveal, the transmitted sound in t e n s i t y used in the determination of the transmission loss had t o be measured on th e re c e iv in g room side o f the reveal where i t merges into the f i l l e r w a ll's surface. The same array of p o in ts was used.

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During t h e measurements the microphone probe was mounted on a mechanical t r a v e r s e system t h a t enabled t h e microphones t o be f i x e d during each measurement i n t e r v a l . I t was then moved by hand from point to p o i n t , although l a t e r developments w i l l i n c l u d e t h e automation o f t h i s t r a v e r s e . All d a t a was s t o r e d on d is k t hrough t h e use o f t h e Remote I n d i c a t i n g Unit ZH 0250 (B & K). In order to avoid r e v e r b e r a n t f i e l d e f f e c t s on the i n t e n s i t y method measurement accuracy t h e t h r e e n o n - p a r a l l e l wa ll s of t h e r e c e i v i n g room were covered with a t h i c k aborbent m a t e r i a l : Conaflex F-400. Naturally this material was removed for the c or re sp o n d i n g sound pressure measurements. 4.2.1

Es ti ma ti on of t h e Phase Er r o r s in t h e I n t e n s i t y Measurements

Given the experimental c o n d i t i o n s d e sc r ib e d above, the r e a c t i v i t y of t h e sound f i e l d was determined in both measurement planes f o r t h e purpose of e s t i m a t i n g the e r r o r s r e s u l t i n g from the phase mismatch between the two measuring c h an n el s . The r e a c t i v i t y i s t h e r a t i o between t h e sound p r e s s u r e and the sound i n t e n s i t y . The e r r o r Le r , defined as the d i f f e r e n c e between the measured intensity from [ 8 ] :

level

(dB)

and the t r u e

intensity

I L = -10 log (1 er 10 p2 rc where Pf e (Pa) i s the sound p r e s s u r e in

level

(dB) can be c a l c u l a t e d P ^

-A . )

* ‘ me

(dB)

(6)

a c ompletely r e a c t i v e f i e l d ,

O

I r e (W/m ) t h e a p p a r e n t , r e s i d u a l i n t e n s i t y a s s o c i a t e d with i t , and P^(Pa) and I

p

(W/m ) r e s p e c t i v e l y , the act ual measured sound p r e s s u r e and

intensity. The measurement e r r o r s as given by (6) were r e l a t i v e l y high (up to 3 dB) a t t h e extreme lower end of t h e f reque nc y range. However, f o r f r e q u e n ­ c i e s equal to or higher than 250 Hz, they were found to be l e s s than 1 dB at 5.08 cm (2") from t h e t e s t panel and l e s s than .5 dB a t t h e r e c e i v i n g room s ide of t he r e v e a l , I f the r e c e i v i n g room had not been l i n e d , the r e a c t i v i t y of the sound would have been h i g h e r , thus i n c r e a s i n g t h e measurement e r r o r s even further.

5. PRELIMINARY TESTS Although t h e use of t h e sound i n t e n s i t y t e c h n i q u e f o r t r a n s m i s s i o n l o s s measurement has been e s t a b l i s h e d by o t h e r s , experimental d e t a i l s of t h e procedure a re s t i l l vague and l e f t up t o t h e u s e r . For example, t y p i ­ c a l l y what r e l a t i o n s h i p e x i s t s between measuring point d i s t a n c e and i n c l u ­ ded r a d i a t i o n s u r f a c e a r e a . The s o l u t i o n s a v a i l a b l e are q u i t e v a r i a b l e

34

Cops [ 5 ] , f o r example uses a mesh s i z e of 19.5 cm x 20. 75 cm with a measurement d i s t a n c e of 4 cm, wh i l e Fahy [4] us e s t h e same d i s t a n c e b u t f o r a mesh of 4.5 cm x 7.5 cm. Th e r e f o r e, b efore the actual t r a n s m i s s i o n loss t e s t s i t was n e ce s s a r y t o determine t h e s e paramet er s e x p e r i m e n t a l l y . During t h e s e measurements only t he t r a n s m i t t e d i n t e n s t i t y l evel was d e t e r mi n e d , t h e i n c i d e n t power b e i n g h e l d c o n s t a n t f o r each o f t h e s e r i e s . 5.1

I n f l u e n c e of Absorbent Material in t h e Reception Room

I n t e n s i t y measurements were made with and without absorbent ma t e r i a l in t h e r e c e p t i o n room. In t h e f or mer c a s e , t h r e e n o n - p a r a l l e l w a l l s o f t h e r e c e p t i o n room were covered with Conaflex F-400. As expected the val ues of the measured t r a n s m i t t e d i n t e n s i t y l e v e l s f o r t h e u n l i n e d c a s e were lower t h a n t h o s e o b t a i n e d in t h e p r e s e n c e o f t h e absor bent ma t e r i a l although t he d i f f e r e n c e s were very small with a maximum d i s c r e p a n c y o f 1 db. However, in o r d e r t o avoid any e f f e c t o f t h e r e v e r b e r a n t f i e l d on t h e measurement accuracy involvi ng sound i n t e n s i t y measurements the r e c e p t i o n room was always l i n e d as d e s c r i b e d . 5.2

Averaging Time

The averaging time i s an important parameter in p r o c e d u r e both f o r acc ur a c y and f o r t o t a l d u r a t i o n o f t e s t . t o minimize time without l os s of accuracy.

a measurement The o b j e c t was

For an a rr a y of 81 p o i n t s t h e t r a n s m i t t e d i n t e n s i t y was measured using d i f f e r e n t l i n e a r a ver aging t i m e s : 4 , 8, 16 and 32 s e c . The r e s u l t s o b t a i n e d were compared with the 32 sec. averaging time which was deemed a c c u r a t e for s t e a d y s t a t e measurement. The i n t e n s i t i e s averaged o v e r t h e t o t a l t e s t p a n e l s s u r f a c e were v e r y s i m i l a r in a l l cases .5 dB). However, a f t e r a comparison of t h e r e s u l t s p o i n t f o r p o i n t , a l i n e a r aver ag i n g t i me of 8 s e c . was chosen s i n c e t h e maximum poi nt d e v i a t i o n from t h e 32 sec. measurement did not exceed ldB. 5.3

Mesh Size

Four d i f f e r e n t mesh s i z e s were t e s t e d 38.1 cm x 38.1 cm (15" x 15"), 22. 86 x 22.86 cm (9" x 9 " ) , 16.33 x 16.33 cm (6.5" x 6 . 5 " ) , 12.7 x 12.7 cm (5" x 5") giving a t o t a l nunber of measuring po i n t s of r e s p e c t i v e l y 3 x 3 ( 9 ) , 5 x 5 (25), 7 x 7 (49), 9 x 9 (81) e v e n l y d i s t r i b u t e d over t h e t e s t p an el 's surface. No attempt to i n c r e as e t h e t o t a l nunber of po i n t s has been made because o f t h e t ime p e n a l t y i n c u r r e d . Each t i me t h e power flow was measured at the c e n t r e of t h e subarea so c r e at e d at a d i s t a n c e , ( t e s t panel t o c e n t r e o f microphone p a i r ) o f h a l f t h e mesh s i z e . The r e s u l t s wi t h r e s p e c t t o t h e aver age t r a n s m i t t d i n t e n s i t y show t h e fo l l o w i n g (Fig. 4):

r... i

___I___I___L

r 1

3B 1cm

x

5B

Icm

MESH

D=:9

□ 5 crr

2

22

x

22

9cm

MESH

D=I 1

Me m

5

IS

Ecm

x

16

Ber n

ME5H

D=9

5cm

M

12

7cm

x

12

7cm

ME5H

D=E

35cm

___L 50 0

FREQUENCY

Fig

4

9cm

1000

(Hz)

T h e I n f l u e n c e o f t h e M e s h S i z e on t h e M e a s u r e d Transmitted Intensity

(tgv = 8 s e c )

L i t t l e difference is seen between the results of the 7 x 7 and 9 x 9 meshes, also with one exception differences were less than .5 dB. For the 5 x 5 mesh, the only large deviation observed was around the coincidence frequence in the 2500 Hy th ir d octave band. The peak in the transmited in te n s ity , which gives rise to the coincidence dip in a trans­ mission loss p l o t , i s seen to be much lower and wider. The r e s u lts o f the 3 x 3 mesh were more ir r e g u la r together with large differences from the smaller mesh sizes. For the present purpose, the smallest mesh size was chosen to avoid inaccuracies and because of the more detailed information possible with respect to establishing the contours of radiated in te n s ity d is tr ib u t io n . 5.4

Measurement Distance

In order to optimize the measurement distance the transmitted in t e n s ity was measured at several distances from the t e s t panel. With regard to the 9 x 9 mesh the distances chosen were ( 1 .5 " ) , 5.08 cm (2" ) s 7.62 cm (3"), 10.16 cm (4") and 12.7 (5"). Certain trends in the results can be observed (Fig. 5).

36

3.81 cm

125

250

500

FREQUENCY Fia. 5

10 00

2000

HOOD

(Hz)

I nfluence of M e a s u r e m e n t Dist anc e 12.7

cm x

12.7

cm Me s h

t av

= 8

sec

When t h e d i s t a n c e i s s mal ler t ha n 10 cm ( 4 " ), t h e r e i s v e r y l i t t l e d i f f e r e n c e (+ .6 dB) between r e s u l t s . However, U i n c r e a s e s with i n c r e a ­ sing d i s t a n c e below c o i nc i denc e . This t re nd r e v e r s e s above c o i nc i d enc e . The l a r g e r t he measurement d i s t a n c e t he l e s s prominent t he c oi ncidence peak, with t h e measured c o i n c i d e n c e f r eq u e n c y f i n a l l y f a l l i n g t o t h e n e xt lower t h i r d octave frequency band. As a consequence t he measurement d i s t a n c e chosen was 5.08 cm ( 2 ") .

6. TRANSMISSION LOSS TESTS 6. 1

Comparison of Standard and I n t e n s i t y - B a s e d Tr ansmissi on Loss Measurement

For t h e sake of comparison between t he two measurement methods and in o r d e r t o t a k e account o f the_ r e v e a l e f f e c t , t h e t r a n s m i t t e d i n t e n s i t y r e p o r t e d in t h i s s e c t i o n was measured on the r e c e p t i o n room side of the reveal.

37

125

350

500

FREQUENCY

Fig. 6

1

PRES5URE

2

I NTENSI TY

1000

2000

4 0 GC

(Hz)

C o m p ari so n be tween S t a n dar d an d I n t e n s i t y - B a s e d T r a n s m i s s i o n Loss M e a s u r e m e n t 12.7 cm x 12 . 7 c m Mesh

t

« 8 sec

As shown i n Fig. 6 f o r th e re ve a l l e f t bare t h e r e s u l t s obtained are g e n e r a ll y ve ry s i m i l a r w ith a maximum d if f e r e n c e o f 2 dB. Greater d i f f e r e n c e s can be seen a t lower fre q u en c ies and t h i s i s p rob a b ly due t o the small re ce p tio n room s iz e . The same trends were observed w ith the re vea l l i n e d . ( V e r a l l , one may conclude t h a t th e e a r l i e r re po rted technique v a l i d i t y , Crocker et al [ 3 ] , Fahy [ 4 ] , and Cops [ 5 ] , has been demonstrated. 6.2

L in in g o f the Reveal

The tr a n s m it t e d s id e o f the r e v e a l .

i n t e n s i t y was again measured on th e r e c e p t io n

room

Fig. 7 shows t h a t th e e f f e c t of lin in g th ic k n e s s increases g r a d u a lly o v e r most o f the frequency range, peaking in e f f e c t between, 1KH and 2KHZ.

125

250

500

1000

FREQUENCY F l 9-

1

Influence

o f L. i n i n p

Transmission

2000

4QQG

the

Reveal

on t h e

Total

Loss

1 2 . 7 c m x 1 2 . 7 c m Mesh

t

av

=8

sec

feneral l y th e t h i c k e r th e l i n i n g , t h e h ig h e r t h e measured tra n s m is s io n l o s s , but only as a f u n c t i o n o f l i n i n g m a te ria l absorption c o e f f i c i e n t as one might expect (see Table 1). L i t t l e e f f e c t i s observed at th e lower frequencies and t h i s i s p robably due to the low frequency absorption c h a r a c t e r i s t i c s o f the l i n i n g m a t e r i a l , although t h e prominence o f g r a z in g mode tra n s m is s io n w i t h r e s p e c t to the l i n i n g at low frequencies might also in flu e n c e t h i s r e s u l t . When the tr a n s m itte d i n t e n s i t y i s measured d i r e c t l y behind the t e s t panel a t a 5 cm (2") d is t a n c e , t h e measured tr a n s m is s io n l o s s i n th e cases when the reveal was lin e d with 0, 2.54 cm (1") or 5.08 cm (2M) were the same, as can be seen in Fig. 8. Thererefore, th e tra n s m is s io n lo s s o f th e t e s t panel alone i s not in flu e n ce d by the l i n i n g , and t h i s suggests th a t the panel v i b r a t i o n is o n l y l o o s e l y coupled t o t h e a ir b o r n modes o f energy t r a n s f e r .

39

H

i 25

i

j ___i___L

i

25D

5 DG

1 DOD

FREQUENCY F i 9-

8

Influence Loss

of

Measured

12.7cm x 12.7cm

8.3

4GOO

(Hz)

Lininq at

2 DOO

the Me s h

the

Reveal

on

the

Transmission

Panel D= 5 . 0 8 c m

(2")

t

= 8 sec

D i s t r i b u t i o n of t h e I n t e n s i t y Radi at ed Through t h e Te s t Panel

This t o p i c has been s t u d i e d t h e o r e t i c a l l y by f o r example Maidanek [7] and r e c e n t l y also e x p e r i me n t a l l y by Fahy [4] using t he sound i n t e n s i t y t echnique» For f r e q u e n c i e s below c o i n c i d e n c e t h e t h e o r e t i c a l model demonst r at es t h a t t he wave p a t t e r n at t he edges of a f i n i t e p l a t e cause most r a d i a t i o n , in c o n t r a s t with an i n f i n i t e p l a t e . It i s f u r t h e r s uggested t h a t f o r t h e s e f r e q u e n c i e s only a s t r i p of p l a t e around the edges r a d i a t e sound power, /bove t h e c o i n c i d e n c e f r e q u e n c y , p a n e l s r a d i a t e from t h e i r whole s u r f a c e , although t he experimental r e s u l t s of Fahy did not c o mp l e t e l y a g r e e wi t h t h i s . The b a s i c experiment ha s been r e p e a t e d ; t h e r a d i a t e d i n t e n s i t y was f o r t h i s purpose measured d i r e c t l y behind the t e s t panel at a d i s t a n c e of 5.08 cm (2") and c o n t o u r s o f equal normal i n t e n s i t y were t h e n p l o t t e d ; r e s u l t s a r e shown at 250 (Fig. 9), and 5000 (Fig. 10). At v e r y low f r e q u e n c i e s ( Fi g. 9) t h e panel i s r a d i a t i n g pred o mi n a n t l y through the c o r n e r s . The i n t e n s i t y t r a n s m i t t e d through a small c e n t e r p o r t i o n o f t h e panel i s much lower and t h e r e f o r e n e g l i g a b l e .

40

V

F ig . 9

Intensity Contours Norme! to t h e Panel

S u rfa ce at 2 5 0 Hz

At mid fre q u en c ies c lo s e r t o th e coincidence frequency i t was found t h a t the center p o rtio n c o n t r ib u t i o n becomes l a r g e r , however greater i n t e n s i t y i s found around th e panel border and th e g ra d ie n ts are found t o be much steeper at the edges than at lower frequency. Closer to and at the co incidence fre q u en cy th e s trong i n t e n s i t y around t h e border o f t h e p la t e remains, but the center p o rtio n o f the panel tends to ra d ia te much more than a t lower fre q u e n c ie s .

Fig. 10 Intensity Contours Wormal to the ^®ns! Surfac© at §@@© Mg

41

Above the coincidence frequency (Figure 10) a q u it e uniform r a d i a t i o n over th e whole s u r fa c e o f th e panel can be observed. 6.4

Fa u lt Finding

The c a p a b i l i t i e s o f the sound i n t e n s i t y technique with regard to the d e t e c t i o n o f c o n s t r u c t i o n o r m a t e r i a l d e f i c i e n c i e s was a ls o examined.

F i® . 11

Ss to om o o f F a u lt

S trip Le n g th

0 .5 cm ;

I n t r o d u c e d by R e m o v i n g

C ra ck W id th

0 .2 m m

W o a th a rs tr ip p in g .

(a p p ro x im a te

d im e n s io n s )

For t h i s purpose a f a u l t was introduced by removal o f th e weatherst r i p p i n g on both sides o f the panel as shown in Fig. 11. The exposed p o r t i o n revealed a crack approxim ately 9.5 cm long and 0.2 mm wide between the panel edge and i t s mounting frame. I n t e n s i t y measurements were made d i r e c t l y behind the t e s t panel at a d is t a n c e o f 5.08 cm ( 2 " ) . The i n t e n s i t y p a t t e r n o b ta in e d was i n v e s t i g a t e d f o r observable i r r e g u l a r i t i e s . I t was found t h a t close to the f a u l t , the values o f tr a n s m i t t e d i n t e n s i t i e s were g e n e r a l l y h ig h e r than a t th e same p o in ts before the f a u l t was introduced. The d if f e r e n c e s in local i n t e n s i t y were s l i g h t a t lower f r e q u e n c i e s , up t o 3dB a t 250 Hz , i n c r e a s in g t o 10 dB at 2000 and f a l l i n g lower again beyond t h i s fre q u en cy. As can be seen in Fig. 12 the e f f e c t i s in d ic a te d by the i n t e n s i t y co n to urs. However, when comparing th e o v e r a l l sound tra n sm is sio n lo ss before and a f t e r the i n t r o d u c t i o n o f the f a u l t (see Figure 13) the i n f l u e n c e o f t h e f a u l t i s o n l y n o t i c e a b l e above 800 H7 and leads t o a maximum d i f f e r e n c e i n o v e r a l l tra n s m is s io n lo s s o f 2.5 dB at 1KHZ w i t h sm a lle r, to n e g l i g a b le d i f f e r e n c e s over th e r e s t o f th e fr equency range. Such a f a u l t could spectrum alone.

ea sily

be overlooked

42

by c o n s id e ra tio n

of the o v e r a ll

LOSS TRANSMI SSI ON

FREQUENCY

Fig. 13

(H z)

Comparison between the Transmission Loss of th e Test Panel before and a f t e r the Introduction o f the Fault 12.7cm x 12.7cm Mesh

t a v = 8 sec

43

D = ^*08cm ^ " )

CONCLUSION

The v a l i d a t i o n o f t h e i n t e n s i t y - b a s e d t r a n s m i s s i o n l o s s measurement has been c o n f i r m e d . A d e t a i l e d measurement p r o c e d u r e has been e s t a b l i s h e d , and t h e a n a l y t i c a l c a p a b i l i t i e s o f t h e new method e x p l o i t e d t o d e t e r m i n e t h e i n f l u e n c e of l i n i n g t h e r e v e a l of t h e t e s t panel with a b s o r b e n t m aterial. The o v e r a l l t r a n s m i s s i o n l o s s has been shown t o i n c r e a s e with increasing thickness of absorbent lining. However t h e intensity measurement t e c h n i q u e i n d i c a t e s t h a t t h e panel r a d i a t i o n i s no t i n f l u e n c e d by t h e p r e s e n c e of t h e l i n i n g , such a c o n c l u s i o n would not have been p o s s i b l e employing t h e s t a n d a r d r e v e r b e r a t i o n room t e c h n i q u e . I t was a l s o d e m o n s t r a t e d t h a t t h e i n t e n s i t y t e c h n i q u e can be used t o i d e n t i f y t h e e x i s t a n c e of untoward sound t r a n s m i s s i o n p a t h s as p a r t of a normal measurement p r o c e d u r e .

ACKNOWLEDGEMENT

This work was s u p p o r t e d from i n d i v i d u a l o p e r a t i n g Na ti o na l S c ie n c e and E n g i n e e r i n g Research C o u n c i l .

grant

A0374

of

the

REFERENCES

[1]

R.W. Guy, A. De Mey, P. S au e r , "The E f f e c t of Some P h y si c al P ar am et er s Upon t h e L a b o r a t o r y Measurement of Sound T ra n s m i s s i o n Los s", C. B.S. Repo rt No. 1 0 5 . , October 1983.

[2]

ANSI/ASTM E90-75, "Laboratory Measurement T r a n s m i s s i o n Loss o f B u i l d i n g P a r t i o n s " .

[ 3]

M.J. C r oc ke r, P.K. Raju, B. F o r s s e n , "Measurement o f T r a n s m i s s i o n Loss of P an el s by t h e D i r e c t D e t e r m i n a t i o n of T r a n s m i t t e d A c o u s t i c I n t e n s i t y " , Noise Control E n g i n e e r i n g , Vol. 17, J u l y - August 1981, p. 6-11.

[4]

F.J. Fahy, "Sound I n t e n s i t y Measurement of P r o c e e d i n g s o f t h e I n s t i t u t e o f A c o u s t i c s , 1982,

[5]

A. Cops, " Ac o u s t i c I n t e n s i t y Measurements and t h e i r A p p l i c a t i o n t o t h e Sound T r a n s m i s s i o n Loss of P an el s and W a l l s " , I n t e r n o i s e 83, P. 567-570.

[6]

M. A. Lang, J.M. Rennie, "Qualification of a 94 Cubic Meter R e v e r b e r a t i o n Room Under ANS S I . 21 " , Noise Contr ol E n g i n e e r i n g / September - October 1981, P. 6 4- 70 .

[7]

R.H. Lyon, G. Maidanik, "Response o f Ribbed P anel s t o A c o u s t i c F i e l d " , J . A . S . A . , 1962, Vol. 34, P. 623.

[8]

Per Rasmussen, "Phase E r r o r s in I n t e n s i t y M e as u re me t s" , B & K A p p l i c a ­ t i o n Note, May 1984.

uu

of

Airborne

Sound

T r a n s m i s s i o n L os s ", P. B5.1 - B5.4.

a Reverberant